Make a seal ring which smoothly sides on the push rod out of thin ridgid plastic, you will have to put a slit in it to place it around the shaft. Make the seal OD large enough to cover the frame opening everywhere as the rod travels. Now make a hat like cover for your ring large enough for for your seal ring to move throughout it's range of movement. You could use thin plywood for this and screw or epoxy this part to the frame rail. This then has a cover ring which captures your seal ring. Visualize a large washer with smaller washer inside(seal ring)its opening then contained by a medium size washer on top. Hope you get the idea? Dave