How to hold custom overflow in place while silicone cures?

[dingo2031]

I got myself a DIY overflow box that I have to silicone into the tank, when I first siliconed it in place it was leaking as well as the bulkheads. I have since removed the box and the bulkheads replaced all my old bulkheads, plumbing in PVC and with pvconline ballvalves for a full siphon and a spare. I need some insight, what grit sandpaper would you recommend "curving" the corner of the overflow as for it goes into the seam

 

[Ranjib]

Clamps?

 

[dingo2031]

Clamps?

Thats what I used the first time, just wanted to make sure

 

[don_chuwish]

Pics? Where'd you get the box?

 

[dingo2031]

Pics? Where'd you get the box?

I have gotten the overflow box installed no issues, The box is from Melevsreef. Off his online shop.

 

[don_chuwish]

Nice.