Should I just add a contingency to the cheapest quote?
A contingency covers cost, but it does not buy back time. If the cheaper contractor is more likely to finish late, the contingency must also cover a probability-weighted allowance for weeks of delay cost, plus a buffer for the tail. Once that allowance is included, the comparison often changes. The worked Illustrative example in this guide shows how.
From the RRD Field Guide, checked against the sources cited in Cheapest Quote vs Delay Risk: The Economics of Choosing a Contractor in Singapore. General information, not advice for your project.
The fuller picture
The cheapest quote is only cheaper if the job finishes on time and on scope. Overrun research shows delays are common and fat-tailed, so a small price saving can be wiped out by a few weeks of rent, lost revenue or holding-over. Compare quotes on expected total cost, including probability-weighted delay cost and the worst plausible outcome, not the tender price alone.