i would suggest buying a baseboard frame of ebay there is a seller that manufactures boards to all dimensions including one specifically for the Hornby track mat although i might suggest buying 2 3x4 baeboards and bolting securely together as this allows you should you wish in the future to have an 8x3 baseboard to expand from. link to seller is here: http://stores.ebay.co.uk/ModelRailwaySolutions?_rdc=1
simon