intermediate the frame members 16 and 17 is at least one second vertical support member 50 each having a plurality of vertically aligned apertures 51 such as universal slots capable of accepting universal fixtures in the embodiments shown two of such members are necessary these vertical supports are preferably made of rectangular or square tubing of a dimension less than the thickness of the frame preferably equal to the thickness of the first support members 30 and have a frontal width 52 which is no greater than twice the width 2x of a first support member each of the second support members 50 are identical in construction and include upper and lower bolts 54 and 56 threaded to the respective top and bottom support member 50 and which operate within appropriate and respective upper and lower guide slots 45 and 46 the respective bolt heads 55 and 57 are insertable in one or more enlarged openings as 47 in fig5 of the cover plates lock nuts 58 and 60 retain the bolts fixed in position the second vertical support members are adapted for horizontal sliding movement see arrows in fig5 and 11 either side of a division line e g one half one third as shown etc between the interior space formed by frame members 12 14 16 and 17 in the embodiment shown this space is divided into three equal spaces the base member 21 39 in fig4 is to show another embodiment including levelling screw 62 associated nut 64 and a foot pad 66 formed as a part of the screw suitable openings 68 are provided for screwdriver or other tool to permit adjustment through the lower frame 14 the openings in all embodiments are.