You really can't cut more shelf into it on the "right hand shooter side". You don't say the weight of the bow. If it's very light, you might have enough strength to withstand the pull, but the bow would be weakened in terms of being susceptible to side bumps... could break right in half.
I picked up a 30# leftie Hill and explored this with Craig at Hill archery and his response was just flat "no way".
Now, there is a simple solution. Any leftie and shoot most right handed, traditional risered longbows if they're willing to adjust a bit.
Some of us deranged longbow fanatics build and shoot longbows with no shelf at all... on either side. It gives us a tremendous advantage in the "right/left" wars... With a properly matched arrow, which would be maybe one, maybe two spine units weaker than usual, a longbow will shoot without a shelf just fine.
I recently made a "traveling bow" for the Howard Hill Longbowmen to hunt with. I wanted it to be usable by both right and left handed members and this is was what I did... no shelf, just a small wedge of leather tucked into the top of the wrap on each side and you shoot off the top of your hand, like when you were a kid.
If you'd like more info and pix of installing the leather wedge in the wrap (you don't have to take the wrap off), just contact me direct:
[email protected]