I have a Hern Mortar, they are not full sized 24 pdr. so a mortar base can be a little smaller.
I used an untreated 8"x8" oak landscape timber. I cut two 26" pieces and bolted them together, I used the 26" length as it looked better than 31", in proportion to the reduced size of the mortar and bed. This gave me a little less than 7 1/2 height and a little less than 15" width. With the bore size of 2.25 In. I felt this size bed would be addiquit. I hollowed out ( for the lack of a better term) the bed for the mortar base and trunion. I used a piece of 3/8" 1 3/4 wide steel I had sitting around for trunion straps. I bolted those through the bed. I then placed the two large bolts trough the bed and placed the handles as shown in Lutz' cohort book.