Do you not think that 14 x12 equipment is over priced compared to standard or commercial i have ten standard national broods ( only three with bees in ) but i recon if i take off the cost of new national stuff it would still cost me about £29 to convert them to 14 x12s (with an eke frames and foundation from thorns) but it would only cost about £13 to commercial with a hamilton converter ( only ten frames but i believe still give you more brood space than a langstroth)
I have a £horns 14x12 EKE and it is at the bottom of a pile of firewood, may not burn it but dont like the version i have
it fits on top of a standard brood box and duplicates the frame runner a, you have to take off the old brood box runners (rendering that useless as i lost the runners in the shed)
so i build my own ekes, they are90mm high and is just B&Q standard square edge 18mm timber, and it sits under the standard brood box, so you use the old frame runners
i have never fixed it to the brood box as the bees just propolise it on, no need for screw but if you wish use flat mirror wall fixings, no lips into the bee space ( though you could put a 12mm sheet and lip to eliminate the joint area bee space if you wish
and i do not even make a 8mm bottom bee space in the eke as it sits on my floor and so what if i get a small amount of brace comb doing a bailley change or combine it is not a problem (nor am i likley to begoing to do double 14x12 brood boxes)
frames, well i don't see that as much cost, as i often do shook swarm or rotate 33% of the frames each year, so the cost is just the difference between DN5 and 14x12 that in £hornes sale is £5 for 50 frames and £7 extra for bigger foundation