Now if you are from the USA you probably know what an egg crate lighting grid is but I'm from the UK and didn't have the foggiest idea! So more trawling and googling later I found an alternative grid in the UK it's a pond filter grid and can be found on eBay for about £7.50 including delivery. I also found a 9L really use box box that would hold all my promarkers with spare slots to accommodate future purchases.
I used my dremel versa-tip heat tool while has 6 different attachments, one of which was the perfect size to slice the plastic grid to cut it down to fit.
links to some of the products I used...