I'm looking to build some ballast boards (gee Ria I wonder where I got that idea from  ) for several different rare fluorescent tubes that I don't have actual fixtures for.
The ballasts are all magnetic and was wondering if anyone could please offer any ideas or suggestions on how to attach the ballasts so the heat doesn't cause problems with the wood board.
Sure isn't the good ol'days when I could get some asbestos felt and attach that to the wood and the heat wouldn't be an issue.

For my thrown together F13T8 fixture I just straight up screwed the ballast in and it was fine. I suppose you could get a bit creative and use some nuts to create a spacer and have an air gap underneath the ballast, promoting cooling.

Asbestos? I've heard that stuff smells really good.....

I'm not extremely electrically inclined, but what you might be able to do is wire in one of those computer fans like this one and mount it to the underside of the board where the ballast is, and then mount some adhesive furniture risers like these to give room for the fan.

Maybe a piece of sheet metal between the ballast and the wood to act as a sort of heat sink much like a fluorescent fixture mounted on a wooden surface could help. The sheet metal would need to be larger than the ballast footprint, however.

I mounted the ballast on the lid of some steel electrical trunking and then screwed this to the board when I made my 250W mercury ballast board, this allowed a sufficient air gap under the ballast to stop the wood getting too hot. You could probably also use a section of cover plate from a scrap fluorescent fitting or a piece of up turned cable tray would do the job.

Probably hard to get now, but for F-can ballasts and other ballasts with the same can width (SL, HO, VHO) Advance made a set of mounting bracket spacers for mounting on surfaces such as wood.

PKG625 However the design should be fairly easy to clone out of metal if you have a drill and a vise to bend the metal.

Another idea would be the mount the ballast on some aluminum extrusion wide enough to accept it for heat sinking and then mount the extrusion spaced from the ballast board by 1/4"-1/2". A scrap coverplate as mentioned in a previous comment also works. Make sure to ground the extrusion though,

Thanks Jamie! Do you guys have cable tray in the US? If so something like this would probably work, turned over and screwed to the board, plus with cable tray rather than trunking lid there is already an assortment of holes to bolt the ballast to.
https://www.cef.co.uk/catalogue/products/4689318-100mm-light-duty-cable-tray-3m-length