Heres a set of plans that I made mine from,only I used 1 x 8 instead of 1x6 and made them 12" inside dim. The key to get them to pull hard is to weight them down ,mine weigh approx 13# each and only stand out of the water approx 1". these do not fold but pull like mules. you might be able to get your current boards to pull harder by adding weight to the bottom of them and changing the location of the pulling point [just a thought]

Don M