Usually rivets from 3/16” diameter through 1/2” are easy to obtain in commonly useful lengths. Custom rivets, for me, are either ordinary diameter rivets in an extraordinary long length or rivets with a larger than ordinary shank diameter.
I recently needed some 3” rivets with a 9/16” shank so I made them from scratch. Vise clamping and peening with torch heat works well but the shank needs to be gripped securely to prevent slipping. Providing a bottom buck works even better and combining the two is a sure thing.
Over the years I have made one style clamping/bucking tool to be used in the post vise which has worked well, That style is shown in images “Rivet upset tool 6 & 8”. The loop depth is about 3” as that is as long as I have ever needed. When making shorter rivets a piece of steel scrap can be inserted to make the bottom stop more shallow. On the 1/4” shank tool I actually tack welded in a stop piece.
After I had finished making a new larger version for the 9/16” shaft rivets I thought perhaps a more universal tool could be fabricated which would have a screw adjustable stop so I drew up a plan. I won’t take the time now to make and test one but if this comes up again I will try it. See “Adjustable Rivet Header”. The shank gripper part is made by sandwiching a piece of card stock between two blocks of mild steel and drilling the hole about the same diameter as the round stock to be used then relieving the sharp edges. The assembly is completed by welding the gripper jaws to the angle flanges and adding the side arms to the fixed nut creating sort of the bottom half of a turnbuckle. I will use 1” thread rod with 1” nuts for the adjustable stop. I might use a coupler nut for the fixed nut.
When welding clamp a piece of slightly oversized stock in the gripper channel so there is a little spring gap built in. The actual stock will fit easily and the vise jaws only have to bend the spring arms a little to grip firmly.
Tip for clean threaded rod end cuts:
Tip for upsetting rivets: