The seat panels alone are surprisingly comfortable - that's the right curve for me - but I need something that works for varying size people and, right now, I need some idea of how seat cushions will affect the leg spacing. I ended up with a thin hard cushion permanently attached to the panels, a removable thin cushion that I take in and out, and a set of removable standard cushions.

For sitting in the plane and thinking about panel layout I used a thin pad glued onto the fiberglass. The thin pad is "Anti-Fatigue Foam Mat", at Harbor Freight for $9 for 4 squares 2'x2'. With these seats being the plans 17" wide, an 18" wide cushion will just fit. For a removable layer on top of that I've used a Home Depot distributed Hampton Bay "sling chair cushion" in "chili stripe".

2/27/21 - 3/12/2021. Refining the cushions. For longer term use I made a set of removable standard cushions, as follows.

Front. Foam is 2" of medium density topped with 1" of memory foam, wrapped with two layers of polyester batting. The foam was a 24" x 48" block, which was sufficient for 2 seats and 2 backs. ($65 at Amazon.) The fabric is blue Olefin, made for outdoor upholstery - I bought a 5.25 yard remnant 54" wide and used 2 yards for the front seats. I like the ridges sewn into the cloth and hope that minimizes slipping. There is an auto seat heating panel under the fabric, both seat and backing, which will tie into a 12v switch on the panel, and join the "Cabin Accessories" circuit. The backing is fiberglass (2 layer BID + peel ply) cast to fit the seat and back of the front seats. I found that Velcro just won't stick to Olefin and the cushions slipped around like crazy. I re-assessed. I turned the seat plate into a foam-BID sandwich panel, with front and rear edges that hold the seat panel. I recast the seat back panel for a height matching the setting in the plane. I did the back panels first - on all 4 edges aluminum strip with Tinnerman clips, fabric wrapped around the strip, then short screws through the 2-BID backplate into the Tinnerman clips. Super light, but not much tension on the fabric. For the seat panels I echoed the plans seat pan, with a foam core sandwiched between 2 layers of BID. On both seat and back I used spray adhesive (3M #77) and for the seat I had foam structure I could use to hold staples. Seat back detail - the pilot side does a nice job of holding itself in place, while the co-pilot side has two pins that push into the seat back.

Rear. 2" medium density foam was cut to the panel size, then a diagonal cut taken at the front and back edge or top and bottom edge, for seat and back panels respectively. Wrapped in 2 layers of polyester batting, then the blue olefin fabric - 1 yard for the 4 panels. The fabric is secured with 3M #77, stapled also. The piano hinge were cut free from the back surface, so a small fabric tab covers the visible part of the hinge. The result looks spartan but is quite comfortable.

Heaters: I went with an automotive heating pad - . The heater is the white sheet shown in photo "seats2.png". For each of the 4 heating pads I routed the plug to the outside, where a handy disconnect meets a 2-pin plug. Both back and seat use a panel mounted rocker switch in the lower outside corner, with high and low settings. When off the draw is zero, at low heat the draw is 0.85A per seat, at high 2.70A per seat. Each side has stand alone wiring, with the switch firing a relay that handles the current draw. Each sides comes with a blade fuse at 10A, saying the manufacturer trusts the wire to that current. I don't have that trust and downsized the fuse to 5 amps. I have panel out for repainting. Once back in the plane I'll install the wiring and switches.