I learned on this plane. If I remember correctly, there was a piece of ply that you slid the tank across as you pushed it into the nose of the plane. Once it got far enough, the tank cleared the ply and it "sprung" up and blocked the tank in. Yours is probably missing this piece. Your best bet would be to use the foam, as mentioned above, to pad the tank and create a tight fit. This is better, actually, than the stock method, as it reduces the chance of vibration causing fuel foaming. The previous owner probably did this, and this is why the piece is missing, and the fit is loose.