Model SL50. It is rated for up to 55 gallons. Dimensions 18x8x16.
There is also the MR75. Rated up to 55 gallons. Dimension 20x8x18.5.
The sump I used was the SL50. I put it in the stand side to side instead of front to back (if that makes sense). I did have to put it in from above instead of through the door.
You generally have to purchase these through a retailer. If you need one, let me know. I am a retailer but I'm not replying to your post for that reason. I typically do maintenance and installations. I am in Springfield, MO.