Under the present conditions of social distancing, you are unlikely to need a backer as, normally, they are to determine any cross shooting, and as the nearest person will be three firing points away, someone will have to be extremely careless to cross shoot.
N.B. Diagrams are for illustration purposes, and are not to scale.
The board size needs to be 1220mm high by 433mm wide and then the board will slot into a frame at the butts. You may need 2 boards, one for 50m. and one for 100yds, and depending which targets you are using will affect how the targets are fixed to the boards. If you’re using the smaller 3 target approach then for 50m, the measurement from bottom of the target holder to the center of each of the three aiming marks is as follows on the left:
If you are using the single aiming marks at 100yds, the bottom to the center of the aiming marks shown on the right.
If you are still using the older style targets, cutting the out the portion that you would shoot through is advised. Allow a little space between the target and elastic, and you should be looking at a round elastic approx. 3mm dia. Using the older targets you will get away with fixing them using Bulldog clips.
Materials you can use for these boards are correx, or, a 5-6mm thick MDF board, but any self supporting material, that can be shot through, would be fine. The MDF weighs more, so is more stable should the wind blow. If you do go for MDF, you may need to take out a circular section of the board behind each aiming mark.
I do hope that this information is of help to everyone. I will update with more information, should it come to light.