Gonna be a tough find to get stock one tho you may get lucky. What I did was make my own in 2017. I ordered a piece of 1/2" HDPE online. I ordered the piece a little longer and wider than needed. I used a piece of cardboard to create a template and cut it down a little at a time so it would be perfect size (leaving 1/8-3/16"" space around it so when vinyl added it will still fit. Then traced the cardboard template onto the HDPE and then cut it out. Think I paid about $60 for the piece including shipping. Link below is where I bought mine, just enter the size and thickness you want (1/2" thick is plenty...I would not go thinner than that personally)
https://www.interstateplastics.com/Hdpe-Marine-Board-White-Sheet-HDPWEM~SH.php
Then had local upholstery shop add the foam, padding and vinyl, think they charged me $90. Turned out really nice. 1st pic is the one they did originally with some extra vinyl the shop had laying around. 2nd pic is after I had it replaced with factory color