faced with having to venture under the house recently for more than one visit - entry through a small gap in the wall, under the deck and the thought off having to crawl the entire time.... I knocked up a set of wheels to take the load off..... they work rather well.

What I used;

4x 7cm M10 Coach bolts, with washers and nuts (Length specific to your build)( see picture)
2x M10 Coach screws (Length specific to your build)
2x lawn mower wheels ( I would suggest small inflatable wheels though for better handling of uneven ground)
1x Rectangle of 1.2cm Ply wood ( mine measures 50cm x 60cm x1.2cm) (picture 1)
1x 80cm length of 75cm x 100cm Frame timber
Socket wrench, extension, and socket ( to fit your nuts)
25mm spade drill bit
2 large old nuts
10mm wood drill
Hammer.
pen

Step 1: Cut Out

Cut your 80cm length of frame timber in halve.
Then by placing the short edge under the ply, and holding your a coach bolt against it - estimate approximately where its going to end. Mark with a line. (Picture 2)
Take the drill and spade bit and drill to this depth + 1.5cm ( to allow for nut) at both ends of the frame timber on the short edge. (creating a counter sink so the bolts dont stick out.) (Picture 3)
Place your ply sheet down on a couple of wooden blocks, and place lengths of frame timber parallel on either side about 1cm out from the edge and sighlty forward of centre.
Replace the spade bit for the 10mm bit and using the centre of your first hole, now drill right through both the frame timber and ply top, to create the bolt holes. (Picture 4)

Step 2: Wheels On

Mark out a chamfer on the ends of the framing timber and cut off - gives a bit of extra clearance.(Picture 5)
Turn the ply upper most, add each coach bolt and using the hammer, strike the top until top of the bolt seats in to the timber ( prevents it from turning when you do up the nut)
Once again turn the ply to under side and add the nuts and washers to the now visible threads on the coach bolts, and tighten up.
Find the approximate centre of each side of the now bolted on framing timber, and using the m 10 drill bit and drill, drill right through the edge.(Picture 6)
Now take your mower wheels, pass a coach screw through the centre as and axle, and thread on a large nut to give clearance, before using the socket wrench screw the coach screws, backing off enough to let the wheel spin freely. (Picture 7)

Step 3: Operation & Ideas

I found the best position for the trolley was laying on my front with the trollies back edge just above my knee caps and taking the weight of my hips, using this method i could largely propelle my self under the house by using the tips of my boots and or hands.
Of course you need ground free of debris and realitvely flat for this to work, you can of course use planks as "rails" if you line them up with the wheel track, and look down at the wheels as you move forward so you dont careen off them in to a foundation stump....
Wheels of the same size, but inflatable instead of hard plastic would make it a much easier to use.... particulary when it comes to small lumps of rubble and the like, which bring the mower wheels to a finger pinching stop. - all though the risk of puncture may come in to play.. not fun when you are at the other end of the work space on your tummy...
Perhaps you would like add a handle to it, to grip in the sticky turns, allthough i didnt as i was then able to just roll off the wheels when i got to the spot where wanted to look up at the floor...