could use corrugated plastic sheet like this for jumps £20 for 3off 8' x 4' sheets
http://www.ebay.co.uk/itm/Correx-Cor...-/250991480140
How about something like this mini profile corrugated roofing for rumble strip. Its 32mm peak to peak so might still be a little deep. Certainly cheap enough.
http://www.wickes.co.uk/invt/240170/?source=123_75