L will work, but its like half a square tube. it will provide half as much bearing surface.
In torsion, each leg of the L will try to separate along half its area and apply compression along the other half.
But a length of angle iron does not resist torsion as well as a smaller diameter square tube.
L is ONE 90 degree break in plane- square is Four 90 degree breaks, Square tube has 4 times the torsional strength of the same weight of L.
Your biggest concern is the transfer of load from the the bolt holes, to the reinforcement.
I assume those are bolt holes.
the weak spot is that the bolt holes have to transfer load thru nothing but the resin. you have multiple bolts- and that helps distribute load somewhat- but failure is still defined by the strength of the resin surrounding those bolts, alone, and not significantly mitigated by the reinforcement.
Again, if bending in plane to the plate is the only force you are resisting, then a single rod or tube inserted may perform fine.
if the plate is being PULLED in plane- then it won’t offer any significant strength.
If it is being twisted axially- the square tube or angle extrusion will be better- but still not offer tensile improvement.
Another solution is to insert something more like rebar. A rod that has crenulations along it full length that you would be filling with epoxy when you inserted it.
But the real question is how much force are you applying?
and what kind?
if you can quantify the design forces you can use the material data sheets to determine which resin and how thick it needs to be to predictably handle the forces involved.