extend a MAC scheme to me unforgeable against unbounded queries

For some prime p we generate two keys $k_{1},k_{2} \leftarrow Z^2_{p}$ where $Z$ is group and the message space of the MAC is also $Z$. We generate a tag for message m with the following function:

$MAC_{k_{1},k_{2}}(m) = k_{1} + m.k_{2}$.

The problem is to extend this construction to be existentially unforgeable under an unbounded number of queries.

It is easy to see that for any adversary, it is almost impossible to forge a tag with only one query, but for two queries:

$a = k_{1} + m_{1}.k_{2}$.

$b = k_{1} + m_{2}.k_{2}$.

it is easy to extract both keys. We can extend this construct to be more secure using hashes or pseudo-random functions but this would only work against a polynomial-time attacker and not an unbounded one. So how can I extend this construct to be secure against and unbounded attacker?